The present invention relates generally to radio frequency (RF) circuit designs, and more particularly to designs implementing skew-symmetrical defected ground structure (DGS) for parallel-coupled line filters.
A parallel-coupled line filter is one of the most commonly used microwave filter designs today because of its simple structure and easy fabrication process, since it does not require via connection to a ground plane. It is essentially a band-pass transmission-line filter composed of a series of half-wavelength resonant conductors in strips that are parallel-coupled, partially electric and partially magnetic, and span the distance of a quarter of one wavelength. This filter can be shorter in length when compared with other types of filters, such as an end-coupled filter.
However, conventional parallel-coupled line filter designs still have several disadvantages. While a conventional parallel-coupled line filter has a length that is shorter than some other types of filters, it still requires too much space in a circuit for today's technology because of its long strips of resonant conductors, which render the process inefficient and costly. Another serious disadvantage of a conventional parallel-coupled line filter is the out-of-band response. This conventional filter has a spurious response exactly twice its center frequency. Since many circuits will generate unwanted harmonics around this doubled frequency band, these unwanted harmonics will be passed through and interfere with the rest of the circuitry.
While a conventional symmetrical DGS can reduce circuit size and shift the spurious response, it may not be applicable to a parallel-coupled line filter since a conventional symmetrical DGS could overlap with adjacent resonators and cause the operation of the line filters to fail.
Desirable in the art of line filter designs are additional designs that improve the reliability thereof.